1. Field of the Disclosure
The present disclosure relates to a device element structure of an electron injection-type avalanche photodiode (photodiode with avalanche multiplication function: hereinafter, simply referred to as APD) that is suitable for a ultra-high speed operation.
2. Discussion of the Background Art
The APD that functions as a Photoreceiver device with high sensitivity is widely introduced in a 10 Gb/s system and the like that uses a long wavelength range (1.5 microns). The typical APD that operates in the long wavelength range is a hole injected APD that uses InP as an avalanche multiplication layer. Almost all the hole injected APDs take a manufacturing process that defines an avalanche multiplication region by Zn heat diffusion to InP. However, it is a serious and technical problem that precise control of the Zn heat diffusion is difficult, and an element manufacturing yield is generally poor.
Meanwhile, an electron injection-type APD that is advantageous in principle, in terms of a high-speed operation and an excess noise characteristic, is known. The electron injection-type APD generally has a structure where InAlAs is used as an avalanche multiplication layer. In the electron injection-type APD, a gain-bandwidth product (GB product) is larger as compared with the hole injected APD, and receiver sensitivity is also superior.
A problem of the electron injection-type APD is that a so-called “guard ring technology” for suppressing edge breakdown around a junction does not reach to completeness in the hole injected APD. This is because it is difficult in the electron injection-type APD to form an “ion implanted guard ring structure” generally used by the hole injected APD, that is, a structure to adjust a depth distribution of acceptor ions such as Be and decrease a multiplication coefficient (increase a breakdown voltage).
For this reason, instead of the “ion injected guard ring structure,” various structures are suggested. For example, a structure where InP is regrown on a side of a mesa of an absorption layer without forming an intended guard ring, a structure where a p electrode layer is formed in a part of a plane of a planar light absorbing layer and an electric field concentration part is disposed on the side of the light absorption layer, and an embedded n electrode structure are known.
FIG. 6 shows an example of an electron injection-type APD that has an embedded n electrode structure according to the related art and shows a cross-section of the embedded n electrode structure where a convex portion is provided in a part of an n electrode disposed on the side of a substrate (refer to Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2005-086109; Patent Document 2: JP-A No. 2007-005697; and Patent Document 4: JP-A No. H08-181349). In the APD of FIG. 6, an n electrode layer 31, an n electrode connecting layer 32, an avalanche multiplication layer 34, an electric field control layer 35, a band gap gradient layer 36, a low-concentration light absorbing layer 37a, a p-type light absorbing layer 37b, a p electrode layer 38, and a p electrode 39 are sequentially stacked, and a light absorbing portion including the low-concentration light absorbing layer 37a and the p-type light absorbing layer 37b forms a mesa shape. On the n electrode layer 31, an n electrode 40 is disposed. The n electrode connecting layer 32 includes an n-type region 32a and a region that surrounds the n-type region 32a and has a low doping concentration. The n-type region 32a is an embedded n-type region. A portion that is shown by a broken line is an electric field concentration portion 21 where an electric field is locally concentrated.
In the light absorbing portion that is obtained by combing the low-concentration light absorbing layer 37a and the p-type light absorbing layer 37b, a band width can be maximized at the same photo-sensitivity by optimizing a thickness ratio of both layers (refer to Patent Document 3: JP-A No. 2005-223022). That is, a device element where a light absorption rate at the same band width is maximized and photo-sensitivity is maximized can be designed. This structure is effective in the electron injection-type APD, but it is difficult to be effective in the hole injection-type APD.
Since the n-type region 32a is disposed on the inner side of the light absorbing portion of the mesa shape, an electric field of a peripheral portion of the light absorbing portion can be decreased, and an electric field of the side and the surface of the mesa is also decreased. For this reason, there is an advantage that the APD of FIG. 6 can confine an avalanche region therein and time degradation of the side and the surface of the mesa can be simultaneously suppressed.
Meanwhile, in the electron injection-type APD that has the embedded n electrode structure, in an operation state, the electric field tends to concentrated (edge electric field) on corners of an outer circumferential portion of the n-type region 32a due to a convex shape of the n-type region 32a. Since the electrical flux line of the edge electric field spreads two-dimensionally, the upper portion side of the avalanche multiplication layer 34 is away from the n-type region 32a and the electric field is likely to be decreased. However, the edge electric field is concentrated on the peripheral portions of the corners of the outer circumferential portion of the n-type region 32a (electric field concentration portion 21). Since electric field dependency of an ionization rate is large, when the electric field of the electric field concentration portion 21 reaches the avalanche multiplication layer 34, the avalanche multiplication layer 34 easily causes a phenomenon of breakdown being generated with the voltage lower than the voltage of an active region of an element center portion, that is, so-called edge breakdown. If the edge breakdown occurs, a sufficiently large avalanche multiplication rate of the active region is not realized, and the difference between the breakdown voltage and the operation voltage decreases. As a result, avalanche excessive noise increases. If the thickness of the low-concentration light absorbing layer 37a increases, an influence of the edge breakdown by the electric field concentration portion 21 increases.
It is known that by controlling the doping profile of the n-type region 32a, in principle, the electric field concentration portion 21 can be suppressed from being infiltrated into the avalanche multiplication layer 34, and the edge breakdown can be suppressed (refer to Patent Document 2: JP-A No. 2007-005697).
However, when the APD is actually manufactured, various process fluctuations are generated. For this reason, it is difficult to control the doping profile of the n-type region of the embedded n electrode structure with high precision, and thus there has been a difficult problem in manufacturing an APD where the generation of edge breakdown is suppressed.